1. Field of the Invention
The present invention relates to a high frequency signal optical transmission system and a high frequency signal optical transmission method, which optically enable a long distance transmission of a high frequency signal (SIN wave) with high stability by compensating a delay phase quantity caused by the transmission.
2. Description of the Related Art
In a long distance coaxial cable, it is impossible to transmit frequencies higher than gigahertz. On the other hand, in an optical fiber, a delay error occurs due to the polarization mode dispersion (PMD) of a light in the transmission of a high frequency signal.
In the case where a microwave signal is transmitted by means of a light, two lights different in wavelength (wavelengths λ1 and λ2) are used. A frequency (phase) difference between those two lights becomes a frequency of the microwave signal to be transmitted.
FIGS. 3A and 3B are diagrams showing two lights different in wavelength which conduct round trip in one optical fiber. Referring to FIGS. 3A and 3B, a left end is an optical transmitting end, and a right end is an optical transmitted side. In this situation, the microwave frequency to be transmitted corresponds to a difference frequency of those two lights. The difference frequency is extracted as the microwave frequency due to the action of a photodetector as a mixer at the transmitted side of the right end. As shown in FIGS. 3A and 3B, the lengths of the round trip of the two lights are different, which represents that the respective delay quantities of the two lights are different due to the polarization mode dispersion (PMD). In the optical fiber transmission, the delay quantity is different between the two lights due to the polarization mode dispersion, and the independent correction is essential.
It is assumed that the respective initial phases of the two lights (wavelengths λ1 and λ2) which are input from the left end of the optical fiber are 0. As shown in FIG. 3B, of those two lights, it is assumed that there is provided an optical phase shifter (phase quantity Φ) for phase control in the optical wavelength λ2. It is assumed that the phase that has been returned from the round trip in the optical wavelength λ1 is ((2 πm)+φ1), and the phase that has been returned from the round trip in the optical wavelength λ2 is ((2 πn)+φ2+2Φ.
In this example, since a light reciprocates in one fiber, it is presumed that the transmitted side of the right end is a middle point of the round trip. In this situation, the optical phase is (φ1)/2 when m is an even, and the optical phase is (φ1)/2+π when m is an odd in the optical wavelength λ1.
Also, the optical phase is (φ2)/2+Φ when n is an even, and the optical phase is (φ2)/2+Φ+π when n is an odd in the optical wavelength λ2.
When the optical phase is controlled so as to meet φ1=φ2+2Φ, the phases of the two lights (wavelengths λ1 and λ2) are conformed to each other at the transmitted side of the right end, or different from each other just by π. Because the signal to be transmitted is transmitted as a phase difference of the two lights (wavelengths λ1 and λ2), the phase of the transmission signal at the transmitted side of the right end of the optical fiber having a long distance is identical with the signal phase at the transmitting end of the left end, or shifted just by π. When the optical phase is controlled to φ1=φ2+2Φ, the long distance transmission of the high frequency signal is stably enabled without caring an influence of the optical fiber. In this situation, the influence given from the external during the transmission in the fiber is commonly given to the two lights (wavelengths λ1 and λ2), a difference between the two lights (wavelengths λ1 and λ2) is obtained at the transmitted side of the right end, and the influence is offset as a common noise.
In order to enable the above operation, it is necessary that a phase shifter that is inserted into only one of the two lights (wavelengths λ1 and λ2) is allowed to pass the light twice in total while the transmission and reception (go and return) light of the round trip is made distinct. In order to achieve the above, a structure is made to provide an optical frequency shifter, a polarization splitter, and a circulator. A polarization state in which two waves of transmission and reception are orthogonal to each other within the optical phase shifter is made for distinction. Also, in order to compensate the polarization after that light has passed the optical phase shifter, the transmission light of the optical phase shifter is reflected, and passed by reciprocation, to thereby remove the polarization rotation of the phase shifter by using the reversibility of the light.
The phase of the two lights (wavelengths λ1 and λ2) due to the round trip is detected by the photodetector according to the principle of a Michelson interferometer as the signal phase of the frequency twice as high as that of the optical frequency shifter at the transmitted side (right end) for splitting the transmission and reception lights. The optical phase shifter is so controlled as to meet φ1=φ2+2Φ, thereby enabling the delay phase compensation.
FIG. 4 is a diagram showing the configuration of a conventional high frequency signal optical transmission system (for example, refer to J. Francois and B. Shillue, “Precision timing control for radio astronomy”, IEEE control systems magazine, 19-27, February 2006). Referring to FIG. 4, the laser beam is distributed by an optical coupler 1 at the transmitting side, and two coherent optical signals that are different in the wavelength are produced from one of the laser beams by a two lightwave generator 2 by the aid of a microwave signal M1. As a result, two optical signals (wavelengths λ1 and λ2) which are apart from each other by the frequency of the microwave signal M1 are produced. The microwave signal M1 is a high-stable signal to be transmitted. The configuration of the two-lightwave generator 2 has the condition in which the polarizations of the two optical signals are conformed to each other.
The two optical signals are vertically polarized waves, which are guided to a polarization beam splitter 3 and pass through a fiber stretcher 4. The two optical signals are further distributed by an optical coupler 6 at the receiving side after passing through an optical fiber 5. One of the two distributed signals is guided to a photodetector 7, and output as a microwave signal M2.
After the other one of the two optical signals which has been distributed by the optical coupler 6 is frequency-shifted by the frequency of a microwave signal M3 by an optical modulator 8 as a round trip signal, and thereafter reflected by a Faraday reflector 9. Since the Faraday reflector 9 gives the Faraday rotation of 90 degrees to the light, and therefore the optical light is reflected as a light that is different in the polarization by 90 degrees.
After the reflected light of the Faraday reflector 9 has been again frequency-shifted by the frequency of the microwave signal M3 by the optical modulator 8, the reflected light passes through the optical coupler 6, the optical fiber 5, and the fiber stretcher 4, and returns to the polarization beam splitter 3. Taking the reversibility of the light into consideration, because the light that has been returned from the receiving side becomes a light different in the polarization by 90 degrees, the light becomes a horizontally polarized wave, and guided to an optical coupler 10 by the polarization beam splitter 3.
In the optical coupler 10, the remaining one of the two optical signals which has been distributed by the optical coupler 1 is mixed with the two optical signals that have been guided by the polarization beam splitter 3. The two optical signals from the polarization beam splitter 3 are different in frequency by twice of the frequency of the microwave signal M3 with respect to the optical signal from the optical coupler 1. The two optical signals that have been mixed by the optical coupler 10 are detected as the beat signal of the microwave by a photodetector 11. The beat frequency is a frequency twice as large as the microwave signal M3. The frequency is multiplied by twice of the shift frequency of a microwave signal M4 by a mixer 12, and an error signal is used for controlling the fiber stretcher 4.
The shift frequency of the microwave signal M3 is to distinguish the transmission signal from the return signal, which is a low-frequency signal. The influence of the microwave signal M3 enters the measurement result because only the phase of one optical signal is measured. For that reason, it is necessary that the microwave signal M3 and twice of the microwave signal M4 of the shift frequency are synchronized in phase with each other through some method. Also, the disturbance that has entered the optical fiber 5 during transmission enters the measurement result since only the phase of one optical signal is measured. In addition, because only the phase of one optical signal is measured, an influence of the polarization mode dispersion (PMD) cannot be removed.
In the conventional high frequency signal optical transmission system described above, since only the phase of one optical signal is measured, the influence of the microwave signal M3 enters the measurement result. Also, the disturbance of the optical fiber during transmission enters the measurement result because only the phase of one optical signal is measured. Further, since only the phase of one optical signal is measured, there arises such a problem that the influence of the polarization mode dispersion cannot be removed.